Capacitive micromachined ultrasonic transducers (cMUTs) are electrostatic actuators/transducers, which are widely used in various applications. Ultrasonic transducers can operate in a variety of media including liquids, solids and gas. Ultrasonic transducers are commonly used for medical imaging for diagnostics and therapy, biochemical imaging, non-destructive evaluation of materials, sonar, communication, proximity sensors, gas flow measurements, in-situ process monitoring, acoustic microscopy, underwater sensing and imaging, and numerous other practical applications. A typical structure of a cMUT is a parallel plate capacitor with a rigid bottom electrode and a movable top electrode residing on or within a flexible membrane, which is used to transmit/accurate (TX) or receive/detect (RX) an acoustic wave in an adjacent medium. A direct current (DC) bias voltage may be applied between the electrodes to deflect the membrane to an optimum position for cMUT operation, usually with the goal of maximizing sensitivity and bandwidth. During transmission an alternating current (AC) signal is applied to the transducer. The alternating electrostatic force between the top electrode and the bottom electrode actuates the membrane in order to deliver acoustic energy into the medium surrounding the cMUT. During reception an impinging acoustic wave causes the membrane to vibrate, thus altering the capacitance between the two electrodes.
One of the important properties of a cMUT is its operation voltage, which is a voltage signal applied to the cMUT in addition to the AC signal applied to generate acoustic energy. In existing cMUT operation methods, a DC voltage is used to bias the cMUT. A TX input signal applied on the cMUT to generate the acoustic output. In these methods, the operation voltage of the cMUT is determined by the DC bias voltage signal only. The same operation voltage level is used in both transmission and reception operations. However, the optimal operating conditions may be different for a cMUT to work in transmission and reception operations. Therefore, using a constant operation voltage level requires a trade-off in selecting a proper operating level in order to obtain an optimal overall performance. This trade-off places a hurdle in a cMUT performance improvement.
To overcome this problem, variable operation voltages in transmission and reception modes have been suggested. This is accomplished by using different bias voltage levels for the two operation modes. Specifically, an AC bias signal with different bias level for TX and RX operations is used to replace a DC bias signal. This method needs two high voltage AC signals in operation: the TX input signal, which is the same as the one used in the other conventional methods, to generate the acoustic output only; and the AC bias signal to change the operation voltage levels between two operation modes. These two high voltage AC signals need to be synchronized. The cMUT elements in a cMUT array cannot share the same AC bias signal for beam-forming. As a result, each cMUT element needs two separate wires in order to operate. This doubles the number of wires used in the cMUT system, and significantly increases the complexity and the cost of the system. The problem is especially acute when a CMUR array with a large number of elements is used.
In order to optimize both RX and TX performances and to simplify the system complexity, better cMUT operation methods need to be developed.